The invention concerns a method for performing magnetic resonance spectroscopy on solid sample containing nuclei of interest with spin quantum number I, comprising: subjecting the sample to a static magnetic field, spinning the sample at the magic angle and broad-band excitation of transverse magnetization of the nuclei of interest.
Related methods are known from [6].
Nitrogen plays structural and functional roles of fundamental importance in proteins and nucleic acids that are essential to many processes in living organisms. Nitrogen-14 is potentially an attractive spectroscopic probe because of its favorable isotopic abundance (99.6%) and reasonable gyromagnetic ratio (˜70% of 15N). However, 14N NMR is not yet a well-established spectroscopic technique. Unlike nuclei with spin S=½ such as 13C and 15N, 14N has a spin I=1 and a nuclear quadrupole moment Q. The interaction of 14N nuclei with local electric field gradients is characterized by a quadrupole coupling constant CQ which can be as large as ˜1-3 MHz. In static powders or crystals, this leads to spectra with a width of several MHz which are difficult to excite uniformly and require broad probe and receiver bandwidths, thus limiting the sensitivity of the experiments. This problem also arises with other nuclei having a nuclear quadrupole moment Q as well as with paramagnetic samples where the spectra of the nuclei are broadened by hyperfine interactions with unpaired electrons.
In solid-state NMR, the first-order components of second-rank tensor interactions (e.g., dipolar couplings, anisotropic chemical shifts and quadrupole interactions) can be averaged out by magic angle spinning (MAS). Nevertheless, even with very fast spinning (νrot>50 kHz), 14N MAS NMR spectra are characterized by a large number of spinning sidebands [1], the envelope of which critically depends on several parameters [2].
In [3-5] application of trains of short pulses are disclosed (DANTE—Delay Alternating with Nutation for Tailored Excitation). In [5] DANTE sequences are used with MAS for selective inversion of a specific sideband family arising from one spin species (i.e. arising from the same sort of nuclei and having the same chemical shift) and selective excitation of a single spinning sideband.
It has been shown in recent years [6-7] that reliable 14N spectra can be obtained through indirect detection via a spy nucleus (typically 1H or 13C) in the manner of heteronuclear single- and multiple-quantum correlation spectroscopy (HSQC and HMQC). So far, these methods have relied on the use of rectangular if pulses applied in the center of the nitrogen spectrum to excite heteronuclear multiple-quantum coherences comprising 14N single- or double-quantum (SQ or DQ) transitions and to reconvert these coherences back into observable single-quantum (SQ) coherences of the spy nuclei. The duration τp of the rectangular excitation and reconversion pulses applied to the 14N channel can be adjusted empirically, typically in the range 0.3 τrot<τp<0.6 τrot. The efficiency of the coherence transfer process depends critically on the optimization of these pulses [2].
It is an object of the present invention to propose a method for performing magnetic resonance spectroscopy using MAS which enables uniform excitation of a great number of spinning sidebands and families of spinning sidebands with different chemical shifts that arise from large first-order quadrupole or hyperfine interactions.
It is a further object of the invention to improve signal intensity.